The present invention relates to signal processing where a common signal component is extracted by processing two homogeneous signals which are extracted substantially simultaneously from one medium and, and more particularly to an improvement of signal processing in a pulse photometer which is used in the medical field, particularly in the diagnosis of the circulatory system.
As a method of separating signal and noise components from two signals which are extracted substantially simultaneously from one medium, various methods have been proposed.
The methods are performed through frequency domain processing and time domain processing.
Also in medical practice, for example, a so-called photoplethysmograph which measures a pulse waveform and a pulse rate, and apparatuses for measuring the concentration of a light-absorbing material contained in the blood, such as an apparatus for measuring the oxygen saturation SpO2, that for measuring the concentration of dyshemoglobin such as carboxyhemoglobin or methemoglobin, and that for measuring the dye concentration injected in the blood are known as a pulse photometer.
Among such apparatuses, an apparatus for measuring the oxygen saturation SpO2 is called a pulse oximeter.
A pulse photometer operates on the following principle. Light beams of plural wavelengths which are different in light absorbency to the target substance are transmitted through or reflected from living tissue, the intensity of reflected or transmitted light is continuously measured, and the concentration of the target substance is obtained from a pulse wave data signal obtained in the measurement.
When noises are mixed into the pulse wave data signal, there is the possibility that the concentration cannot be correctly calculated and erroneous treatment may be caused.
In order to reduce the noise level in a pulse photometer, methods such as that where the frequency band is divided and attention is paid on signal components, and that where a correlation between two signals is determined have been proposed. However, the methods have a problem that the analysis requires a prolonged time period.
Therefore, the assignee of the present invention has proposed an art in Japanese Patent No. 3,270,917 where light beams having two different wavelengths impinge on living tissue, two pulse wave signals are obtained from transmitted light beams, a graph is formed while using the level of the two pulse wave signals as the ordinate and abscissa, respectively to obtain a regression line, and the oxygen saturation of arterial blood or the concentration of a light-absorbing material is obtained on the basis of the gradient of the regression line. According to the related art, the measurement accuracy is improved, and the power consumption is reduced. In order to obtain a regression line or the gradient thereof by using many sampling data with respect to pulse wave signals of the wavelengths, however, a large computation processing power is required.
Furthermore, the assignee of the present invention has proposed a method in JP-A-2003-135434 where, although a frequency analysis is used, a pulse wave signal itself is not extracted in the analysis unlike a related art, but fundamental frequency of the pulse wave signal, and the pulse wave signal is filtered by using a filter using harmonic frequencies thereof to enhance the accuracy.
In the use at home, particularly, it is supposed that a pulse oximeter is used in various manners. Therefore, a wide variety of artifacts exist, and a higher anti-artifact property is requested in the case of SpO2 which is used in a hospital.
When an artifact is contained, the measurement system is disturbed, and there is a case where SpO2 is erroneously displayed.
A typical artifact is body motion. For example, such an artifact is caused by a phenomenon where a probe which is attached to the subject to be measured is moved by body motion, and the optical path between the light source and a light-receiving face is changed, or living tissue is deformed by a force applied to the tissue.
In the case of a neonatal infant or an infant child, particularly, an artifact is often contained, and a motion of a hand or a foot, bitter sobbing, shiver, cough, and the like function as an artifact source.